The present invention relates to a pattern forming apparatus for forming a very fine device pattern, such as a semiconductor IC circuit pattern, on a sample, such as a semiconductor wafer and pattern transferring mask, with the use of a beam such as a charged beam.
With a recent high integration of the LSI, a corresponding circuit line's width has been becoming finer and finer in a corresponding semiconductor device. Such semiconductor devices are formed by transferring drawing patterns, that is, desired circuit patterns corresponding to several tens of original patterns, on a mask such as a reticle, sequentially to light exposure areas on a wafer through a high-accurate alignment and performing chemical processing such as etching. Here, as a transferring device use is made of a stepper having a high accurate optical system. The wafer is held in place on a high accurate XY stage so as to allow a whole wafer surface on the transferring side to be exposed with light. The wafer is moved in a step & repeat fashion relative to the optical system and such a transferring apparatus is called a stepper.
The original pattern is formed as a Cr pattern on a glass substrate finished with high accuracy. Upon pattern formation, a glass substrate with Cr evaporated on one side is uniformly coated with a resist and the whole substrate surface is scanned with an energy beam, such as a focusing electron beam or laser beam, in accordance with design data to make a desired area of the resist photosensitized. And the photosensitized resist is developed to provide a resist pattern and then the Cr is etched with the use of the resist pattern to obtain a desired pattern. Since the pattern is formed with the focused spots joined, it is possible to form a high accurate pattern depending upon how the beam is controlled.
The Cr pattern formed on the glass substrate with the use of such a pattern drawing apparatus need be aligned as a high accurate overlay on a circuit pattern on the wafer, so that a pattern transfer is carried out.
However, there are sometimes the cases where the wafer itself may be deformed with a progress of its processing. In order to realize such a pattern transfer process with such high accurate overlay relative to the circuit pattern on the deformed wafer, it is necessary that the circuit pattern on the glass substrate be displaced to achieve an alignment with the deformed wafer.
In the case where a first transfer process of the circuit pattern on the wafer has been made with the use of a deformed-pattern-formed glass substrate, it followed that, in order to achieve a high accurate overlay, it is necessary that a pattern on the glass substrate used at a second and subsequent transfer processes has to be set in a deformed sate as in the case of the circuit pattern used in the first transfer process.
As the method for displacing such a drawing position use is made of a method by which a correction amount at a given point (X, Y) is defined by a function and found through its calculation as, for example, in JPN PAT APPLN KOKAI PUBLICATION NO. 7-52948.
Since, however, the correction value in a given coordinate on a whole drawing surface is calculated, there arises the following problem.
In FIG. 1, A represents a grid point showing an ideal drawing position and B a grid point given by correction. If, as shown in FIG. 1, a correction amount is locally greater in a portion of the drawing area on the whole drawing section 7, it is difficult to represent such correction amount, so that correction cannot be achieved with high accuracy. If a higher-degree polynomial equation is used as a function so as to make its approximate precision higher, then the calculation procedure of the correction equation becomes vast and complicated.
Thus, in the case where the whole drawing section of a sample is to be corrected in such a conventional method, for example, the correction amount is locally greater in a portion of the correction area, it has been difficult to make correction with high precision.